Mind Hacks wrote about a 2003 study from Epilepsy and Behavior about ecstatic seizures.
The first seizure occurred during a concert when he was a teenager. He remembers perceiving short moments of an indefinable feeling. Such episodes recurred and a few months later evolved into a GTC [generalized tonic–clonic seizure]. He characterizes these sensations as "a trance of pleasure." "It is like an emotional wave striking me again and again. I feel compelled to obey a sort of phenomenon. These sensations are outside the spectrum of what I ever have experienced outside a seizure." He also describes cold shivering, increased muscle tension, and a delicious taste, and he swallows repeatedly. He enjoys the sensations and is absorbed in them in a way that he can barely hear when spoken to. When in a particular, relaxed mood, he can sometimes induce seizures by "opening up mentally" and contracting muscles. He denies any religious aspects of the symptoms. "It's the phenomenon, the feeling, the fit taking control." It lasts a few minutes and afterward he is tired with difficulties expressing himself for about 1 hour.
They also ran this quote from Dostoyevsky, who said the following about his own epilepsy seizures:
"I would experience such joy as would be inconceivable in ordinary life – such joy that no one else could have any notion of. I would feel the most complete harmony in myself and in the whole world and this feeling was so strong and sweet that for a few seconds of such bliss I would give ten or more years of my life, even my whole life perhaps."